If you keep them clean and stored properly you should get years out of a line(under normal use). I only change one if it starts to crack, split, gets stiff or shows real signs of wear/fatigue.
Or if I just want something new.

Heat and sunlight are the typical 'line killers.' The back window of your car during the summer is the kiss of death. Another potential problem is leaving your line tightly wound on a reel (light lines are the worst). If you're not going to use the reel for an extended period pull the line off the reel, make loose coils and stick in a gallon zip lock bag.

Frequently you don't even have to remove the reel it will probably fit in the bag as well.

What I've done with my lines over the years is every two or three outings I'll strip all the line off the reel and give it a good cleaning in the sink. I keep them out of the light, when not in use things are stored in the cool bsmt. out of the light. I've got three or four lines older than ten years and I know my Wulff bass taper line is at least 13-14 yrs old. If they still float and haven't cracked I'm using em.

What I've done with my lines over the years is every two or three outings I'll strip all the line off the reel and give it a good cleaning in the sink. I keep them out of the light, when not in use things are stored in the cool bsmt. out of the light. I've got three or four lines older than ten years and I know my Wulff bass taper line is at least 13-14 yrs old. If they still float and haven't cracked I'm using em.

That's what I do. I probably have 12-15 fly lines, one my Cortland 333 Rocket Taper eight weight is 40 years old. I just can't bring myself to throw them away when they still work, no cracks and don't have memory coils. I should remove them from the reels like Fred suggest, but all mine are stored on the same spool year after year. I guess the fly line companies know what they are doing.

The biggest killer for me is driveway casting -- but I'm aware of what will happen and just use lines which are less expensive for this. I fish every weekend and fairly regularly during the week and probably put a lot of hard use on my lines. I also fish in areas where my lines get dragged through sediment often.

I got 1.5 years out of my most-used 5-weight line before I had enough cracking and random dings to require a new one. I'm cool with that -

I have a bunch of lines in the 15-20 year range, and they still work great. I clean them regularly and I never cast them on anything but water ( I have a dedicated practice line). I wondered if I should get rid of them cecause of age but I see no reason to do that, they perform very well.

On another BB, we were discussing why the tips of fly lines begin to sink.

I did an experiment. I got an old 4 wt fly line, actually my original 4 wt fly line from the 1980's. It had multiple cracks in the first foot so water could enter the braided core in multiple places. I fished it for an hour and the first foot sank under the water.

Then I coated that part of the line with a thick silicone fly paste and line treatment. The treatment coated the line and filled in the cracks. The line then floated. Silicone fly floatant is hydrophobic and when to coated the line, it allowed the surface tension to float the line.

Modern fly lines are supposed to renew their plasticizers and hydrophobic coating by slowly releasing it from the line coating and keeping the line slick and water repellant (hydrophobic). When these are used up, the fly lines coatings become brittle and crack.

The reason even new fly line sink is that they get dirty and the coating is no longer next to the water. The dirt on the coating is. So they sink. Clean the line and it will float again or treat the line with line treatment.

"Believe it or not, a fly line can get dirty from water as well. Dirt, algae and dust that are dissolved in water will adhere to the outer skin of a fly line. This is why we make most of our fly lines hydrophobic - they repel water. After wiping the line clean, apply a light coating of a fly line dressing. This will help to re-lubricate the line and gives the fly line a slick and clean finish."

"Within the balance of the coating are a number of chemicals that essentially act as moisturizers. These chemicals keep the plastic coating supple and strong and prevent it from drying out, or de-plasticizing. The course of time and ultra-violet rays has an aging process that will eventually make the fly lines dry out and crack."